1. Field of the Invention
This invention relates generally to highway striping and marking materials and specifically to retroreflective beads used in highway striping and marking materials to enhance visibility of the highway, where striped and marked, especially after sundown.
2. Description of Prior Art
Pavement markings such as paints, tapes, and individually mounted articles to guide and direct motorists traveling along a roadway are known. During daylight the markings may be sufficiently illuminated by ambient light to effectively signal and guide motorists. At night, especially when the primary source of illumination is the vehicle's headlights, the markings may be insufficient to guide adequately motorists because light from vehicle headlights hits the pavement and pavement markings at a very low angle of incidence, with the result that the light largely reflects away from the vehicle.
Retroreflection is the mechanism whereby light incident on a surface is reflected in a way that much of the incident light is directed back towards the light source. The most common retroreflective pavement markings, such as lane lines on roadways, are made by dropping transparent glass or ceramic optical elements onto a freshly painted line to which the optical elements adhere and desirably become partially embedded therein. Each transparent optical element preferably acts as a spherical lens. Incident light desirably passes through the optical elements to the pavement marking paint or sheet, striking any pigment particles embedded therein. The pigment particles scatter the light. The scattered light includes a portion that is directed back into the optical element, which then redirects that portion of light back towards the light source.
In addition to providing desired optical effects, pavement markings must withstand road traffic, and adverse weather conditions, and are subject to cost constraints in the course of manufacture and installation.
Vertically oriented or upwardly disposed surfaces provide good orientation for retroreflection. As a result, attempts have been made to incorporate vertical surfaces in pavement markings, such as by providing protrusions in the marking surface. Vertical surfaces may advantageously prevent build-up of water over the retroreflective surface during rainy weather, which water would otherwise interfere with retroreflection.
Sometimes raised pavement markers are placed at intervals along a pavement marking line as disclosed in U.S. Pat. Nos. 3,292,507 and 4,875,798. These markers are relatively large, generally being several centimeters in width, and five to twenty millimeters in height. Typically, such markers require assembly of different components, some of which were previously individually molded or cast. Therefore, such markers are relatively expensive to manufacture. The size of the markers makes them subject to substantial impact forces from passing vehicles. As a result, the markers must be well secured to the pavement, increasing installation costs and also removal costs when the markers must be replaced. Moreover, because the markers are applied at intervals, the reflected light provided by the markers are in the form of discontinuous spots of light. A continuous bright line of reflective light is more desirable.
Embossed pavement marking tapes, as disclosed in U.S. Pat. Nos. 4,069,281 and 5,417,515, represent an approach that has been taken towards providing better highway marking surfaces. Selective placement of transparent optical elements on the sides of embossed raised protrusions present in the tapes results in somewhat effective marking. However, such tapes are relatively expensive compared to painted markings and tend to pick up dirt that adheres to the tape even after a hard rain. As a result, tape usage is often limited lower traffic areas such as unlighted intersections and railway crossings. These embossed tapes are constructed with polymeric materials which are susceptible to wear.
Yet another approach to providing retroreflection is the composite retroreflective element such as disclosed in U.S. Pat. Nos. 3,254,563 and 4,983,458. These retroreflective elements essentially have a core with optical elements embedded in the core surface. Some also contain optical elements dispersed throughout the core that become exposed with wear. The core may be irregular in shape or may be shaped as a sphere, tetrahedron, disc, square, tile, etc.
Some known retroreflective elements have centers formed of polymeric cores or binders. A pigmented core or binder may serve as a diffuse reflector, allowing spherical optical elements to be used on horizontal and vertical surfaces. Other constructions have transparent optical elements including specular reflectors such as metallic silver. The metallic surface directs light back towards the light source. In such cases pigmented cores are not used. Geometry of the optics may make a specular coated optical element less effective when embedded in pavement marking paint on a horizontal surface, and more effective when embedded in the vertical surfaces of a retroreflective support element.
Another retroreflective element construction, disclosed in U.S. Pat. No. 3,252,376, uses silvered glass flakes as a specular reflector on the surface of a spherical polymeric core; no spherical optical elements are used.
Still another known construction disclosed in U.S. Pat. Nos. 4,072,403; 4,652,172 and 5,268,789 has a retroreflective element with a plastic globule refracting incident light onto a layer of glass optical elements attached to the bottom of the globule. The glass optical elements focus light onto a specular coating or film located below the elements. Incident light is then reflected back along the original path towards the source.
Shaped polymeric retroreflective elements with pigmented cores and glass optical elements embedded in the vertical surfaces are disclosed in U.S. Pat. No. 3,418,896. These retroreflective elements are formed by extruding pigmented polymer into rods of different cross-sectional shapes. Glass optical elements are embedded into the surface of the polymer before it hardens, then the rods are sliced to form the elements.
Polymeric retroreflective elements are undesirably susceptible to wear, especially in high traffic regions, and to degradation due to weathering.
In an attempt to overcome these limitations, retroreflective elements have been constructed having a ceramic core surrounded by optical glass with a metallic specular coating.
One such construction approach is a rock or glass sphere core as disclosed in U.S. Pat. Nos. 3,043,196 and 3,175,935, covered by a polymeric binder with glass optical elements having a specular metallic coating embedded in the polymeric coating.
Another construction disclosed in U.S. Pat. No. 3,556,637 includes a glass sphere and a layer of glass optical elements attached to the bottom of the glass sphere with a polymeric binder. A metallic film below the glass optical elements acts as a specular reflector. However, these glass sphere-metallic film combinations do not provide the brilliant white or yellow color for the reflective light that is desirable in a high quality highway machine and is necessary to meet the requirements of many regulatory authorities. The glass sphere-metallic film approach tends to produce reflected light that is grey or silver, instead of white, or that is bronze or gold, instead of yellow. None of these are acceptable.
Other constructions include a composite lens element serving both as a retroreflective element and a skid-resistant particle as disclosed in European patent 0,322,671. The skid-resistant particle, which acts as a core, may be either a corundum particle or glass sphere, and is coated with a pigmented polymeric binder acting as a diffuse reflector.
A ceramic element having glass optical elements embedded throughout a glass core and at the core surface is disclosed in U.S. Pat. No. 3,171,827. A thin metallic film separates the optical elements and the glass core to provide an efficient specular retroreflective system. Alternatively, optical elements having a refractive index greater than 2.0 are used. These high refractive index optical elements are asserted as being capable of reflecting light without the need for a reflective backing.
A ceramic retroreflective composite element having a transparent glass sphere with smaller glass optical elements embedded in the surface is disclosed in U.S. Pat. Nos. 3,274,888 and 3,486,952. A thin metallic film separates the optical elements and the glass sphere to provide specular retroreflective system. The elements are formed by first coating the glass spheres with metallized optical elements using a temporary polymeric binder. The coated spheres are then tumbled with excess optical elements in a rotary kiln. When temperature exceeds the softening temperature of the glass spheres, the optical elements embed themselves into the surface of the spheres. Later the film is etched away from the exposed portions of the optical elements.
WO 97/28471 discloses a retroreflective element having an opacified ceramic core and ceramic optical elements partially embedded in the core. The diffuse reflecting ceramic core, in combination with the transparent optical elements embedded in the surface, provides a retroreflective element asserted to be without the gray coloration and the susceptibility to corrosion associated with metallic specular reflectors. Although such all-ceramic retroreflective elements are asserted to have improved resistance to wear and weathering, crush-resistance remains a problem, limiting life of the retroreflective element.
Beads for highway marking are disclosed in U.S. Pat. Nos. 3,171,827 and 3,274,888. '827 discloses a bead with a polymer core, while '888 discloses beads consisting of all glass bonded to a glass core that is fabricated by melting these glass components together; there is no polymer core nor is any adhesive used.
U.S. Pat. No. 4,983,458 discloses a bead similar to that disclosed in '827, but having a tetrahedron shape intended to increase adhesion to highway marking tape. However, the '458 product does not function well with current reflective bead application equipment.
U.S. Pat. No. 5,456,546 discloses use of single glass beads in highway marking lines.
U.S. Pat. Nos. 5,774,265 and 5,917,652 disclose retroreflective elements. The elements are not white or yellow, which is desirable since white and yellow are the colors used for highway striping and are, in many instances, required by law. The retroreflective elements disclosed in '265 and '652 are expensive to manufacture.
U.S. Pat. No. 5,942,280 discloses a bead that is a variant of the beads disclosed in the '458 and '827 patents discussed above.
United States published applications 2005/0100709 and 2005/0158461 disclose beads that are similar to the beads disclosed in the '458; '827 and '546 patents discussed above, but the beads disclosed in these two pending patent applications are not all ceramic.
United States published application 2005/0001342 discloses an agglomeration of small beads without a core, manufactured by a slow, costly method.